Crop growers loathe pigweed because its biology and resistance to herbicides render it extremely difficult to control. With the appearance first identified in South Africa in 2018, this well-known weed has a negative impact on regional food production.
Ingrid Boshoff, technical lead of herbicides at Syngenta South Africa, explains how early, quick, and effective action may be performed in order to control and remove this weed.
What is pigweed?
Amaranthus palmeri is an aggressive competitor that grows fast and propagates extremely effectively. The weed is dioecious, with separate male and female plants. Under favourable growing conditions, it can maintain a growth rate of up to 6cm per day, and it is possible for a single female plant to produce up to 600 000 tiny, easily dispersible seeds. The weed can also reproduce vegetatively through regrowth from stalks left behind in fields.
“Palmar amaranth or amaranthus palmeri is a type of pigweed that is rapidly spreading throughout the world. Since its establishment in the USA, it has become our country’s most troublesome weed. The reason why it is so problematic is because it’s become very difficult to control,” explains Boshoff.
“Not only is palmar amaranth very good at spreading and establishing itself but it has an incredible ability to develop resistance. Worldwide it has developed herbicide resistance to more herbicide groups than any other weed.”
Impact on crops
According to a study on herbicides that was done in South Africa in 2022, nine of the 17 herbicide groups were confirmed internationally. In South Africa, resistance to two groups out of the 17 groups was reported.
“An academic paper was published last year about the South African population in Douglas and it confirmed resistance within these two groups or modes of action. Group 2 is the acetolactate synthase (ALS) inhibitor and contains a multitude of commonly used herbicide actives.
“And then group 9 refers to glyphosate and the South African farming community heavily relies on glyphosate with the production of soybeans, cotton, and maize, so this is of great concern,” Boshoff explains.
How to identify pigweed among crops
Boshoff says pigweed has some defining characteristics which can help with identification. The most unique characteristic is that the leaf petiole is longer than the leaf itself.
“This can be especially helpful in early season identification. Just take the leaf petiole and fold the leaf over it, if it sticks out beyond the leaf blade it is worth investigating further. Later in the season when the plants are flowering, you will see two distinctively different flowering stalks,” she says.
Steps to successfully control the weed
The answer to controlling pigweed successfully lies in a holistic and integrated approach. Such a programme must include the sanitation of all implements, equipment, clothing and shoes by washing everything in one central spot.
Follow these tips:
- Firstly, use a good chemical programme, always start clean and keep it clean.
- A good pre-emergence herbicide programme is very important in the control of this weed.
- This is followed by a post-emergence spray to keep that control throughout the season.
- In these sprays, one should use mixes of different ai’s in different herbicide groups to fight resistance buildup.
“Deep cultivation is recommended for infested fields as palmar amaranth can also reproduce vegetatively. Which is why it is also important to burn any plant residue when manually removing large plants from a field,” she advises.
Furthermore, she emphasises the sanitation of implements, especially where contractors are used for harvesting. The seeds are so small and can easily hitch a ride on a harvester.
Advice to farmers
The most important factor in the battle against this weed is really keeping track of it; where it is and where it’s going. This helps any farmer navigate the potential impact of the pigweed.
Farmers are encouraged to seek help if there is any suspicion that this pigweed has encroached on their crop.
At the end of the day, farmers and they should know that they are not alone. Boshoff, cannot stress enough that there is help out there.
“While the industry and academia are very busy with this and conducting research on how to control it, we need the collaboration of our farmers. Populations must be reported so that we can help you control them, that’s how we’ll win this war on palmer amaranth. So, I encourage anyone that suspects they may have it on their farm to contact me directly,” she says.
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